The Google dictionary defines 'best' as "that which is the most excellent, outstanding, or desirable." In that case, there's no linebacker unit that is more 'excellent, outstanding or desirable' than the one in Dallas.
The Cowboys are no strangers to boasting a top position group in the NFL. Even though injuries hampered it last year, the Dallas offensive line has been regarded as the consensus best in the NFL for stretches in recent history. Buoyed by All-Pros Travis Frederick, Zack Martin and Tyron Smith (who are all younger than 29 years old), the Cowboys not only had the talent to boast the top offensive line in the NFL, but they also had youth on their side, which gave them staying power.